The Coastal Subbasin includes the watershed area along the SLR River from its mouth at the Pacific Ocean to RM 19 at Rice Canyon, approximately one mile east of Interstate 15 (Figure 2). Stream elevations range from sea level at the mouth of the SLR River to approximately 1,600 feet in the headwaters of the tributaries. While it is the third largest subbasin of the SLR Basin at 102 square miles, 18% of the total watershed, it is easily the most populated. This assessment area encompasses the dominant population centers in the basin area with the cities of Oceanside, Vista, Bonsall, and portions of Fallbrook residing within its boundaries. The lower and southern portions of the subbasin are mostly urban/residential, commercial, and light industrial areas. The northern portions of the basin are held in larger private parcels managed for agricultural crop production. California Highway 76 runs parallel to the river throughout the subbasin.

As a result of population growth in the subbasin the river, its stream channel and the surrounding riparian have been the most altered and managed area in the basin. The channelization of the river, manipulation of riparian vegetation, draining and downsizing the estuary, and utilizing numerous wells throughout the subbasin have cumulatively changed the form, function, and habitat of the river and its estuary.

SLR River estuary following the removal of the Pacific Street Bridge, December 31, 2008.